The present invention relates to an improved electric actuator. The apparatus may find application as part of any control system utilizing mechanical motion or position as an output but is particularly suited for use in automotive air conditioning systems.
Conventional automotive air conditioning systems normally utilize an actuator of the vacuum type. These actuators are switch operated devices which use the vacuum generated by the automobile's engine to effect mechanical motion of final control elements. Vacuum-type actuators are not expensive, provide a reasonable amount of power, and are inherently protected from burnout. Advances in automotive air conditioning technology, however, have created a need for an actuator with features a vacuum-type actuator is not able to provide. Modern, sophisticated automotive air conditioning systems now demand actuators capable of multi-position or infinite position actuation. Vacuum-type actuators, however, are on-off devices and are only capable of two position operation. Additionally, vacuum-type actuators require hoses to be run from the vacuum source to the actuator. As modern automotive air conditioning systems can require the use of five or more actuators, the employment of multiple vacuum-type actuators can be quite cumbersome.
Accordingly, electric actuators have been used in modern automotive air conditioning systems to overcome the problems enumerated above. Conventional electric actuators, however, also have a number of disadvantages in this application. Two-position electric actuators must employ limit switches which shut off power to the unit when the final mechanical output element reaches the end of its range of travel. Limit switches are necessary since a stalled electric motor will draw enough current to present a real danger of fire. Utilizing limit switches in the design, however, means that the range of travel of each actuator must be matched to its physical environment. In an air conditioning system requiring multiple actuators, each with a different range of motion requirement owing to different physical environments, this means that each actuator must be individually designed with limit switches appropriately located and calibrated. This adds time and expense to the design and manufacturing process. Another problem arises when using conventional electric actuators to provide multi-position actuation since a special segmented switch design must be employed to define each intended position. Conventional electric actuators in a multi-position application also require an additional control wire for each additional position which the actuator provides. This adds complexity to the design and manufacturing process requires that additional inventory be maintained, and increases the total cost of the system.
The apparatus which is the present invention is an improved electric actuator which overcomes the disadvantages of conventional electric actuators in automotive air conditioning applications. The apparatus can function as a two position, multiposition, or infinite position actuator while employing only three wires to control its movement. No limit switches are required as the unit is inherently protected against stalls. Furthermore, the apparatus defines its own range of movement according to the physical environment in which it is placed. Therefore, a single actuator design may be used throughout the system in an application requiring multiple actuators.